1. Field of the Invention
This invention relates to binary data transmission systems wherein the bit rate of the binary data exceeds that at which intersymbol interference would occur. More particularly, the invention relates to method and apparatus for digitally converting a binary signal into a modified duobinary signal.
2. Description of the Prior Art
Duobinary systems are explained, for example in an article appearing in IEEE Transactions on Communications and Electronics, vol. 82, May 1963, pp. 214 - 218, as well as a variety of generally available publications. In particular, the duobinary system is disclosed and claimed in U.S. Pat. No. 3,238,299 entitled, "High-Speed Data Transmission System" by the present inventor. Subsequent publications by the present inventor have disclosed a number of variations in the basic duobinary concepts. The duobinary technique, as originally developed, contemplated a baseband three-level, correlative level-coded signal in which the amplitude of the wave increased with decreasing frequency, reaching a maximum value at a frequency which was essentially zero. Important among the additional publications of interest is the article entitled, "Correlative Digital Communication Techniques", appearing in IEEE Transactions on Communications Technology, vol. 13, June 1965, pp. 203 - 208. In addition, a number of correlative level-coded techniques are described in U.S. Pat. No. 3,388,330 entitled, "Partial Response Multilevel Data System". More particularly, a technique for converting a binary signal into a modified duobinary signal and the particularities of the characteristics of the modified duobinary signal are described in considerable detail in U.S. Pat. No. 3,457,510 entitled, "Modified Duobinary Data Transmission" by the present inventor. U.S. Pat. No. 3,457,510 is incorporated herein by reference.
It is important to note first that the conversion filter of the subject U.S. Pat. No. 3,457,510 comprises a bandpass filter having zero transmission at zero frequency and at 2f.sub.1 Hz. The maximum transmission occurs at the frequency f.sub.1 Hz. Further, the conversion of the correlative two-level signal is accomplished by filtering or shaping of the two-level signal. In contrast, the correlative level-coded modified duobinary waveform of the present invention is accomplished solely by means of digital techniques.